This invention relates to a fluid displacement apparatus, and more particularly, to a scroll type fluid displacement apparatus of the type which may be used as a compressor, expander or pump.
Scroll type fluid displacement apparatus are well known in the prior art. For example, U.S. Pat. No. 801,182 issued to Creux discloses a scroll type apparatus which includes two scroll members, each having a circular end plate and a spiroidal or involute element. The scroll members are maintained angularly and radially offset so that both spiral elements interfit to form a plurality of line contacts between their spiral curved surfaces to seal off and define, along the axial seals between the end plates and the axial ends of the adjacent spiral elements, at least one pair of fluid pockets. The relative orbital motion of the two scroll members shifts the line contacts along the spiral curved surfaces and, as a result, the volume of the fluid pockets increases or decreases, dependent on the direction of the orbital motion. Thus, a scroll type fluid displacement apparatus may be used to compress, expand or pump fluids.
The principles of operation of a conventional scroll type compressor will now be described with reference to FIGS. 1a-1d. These figures show the relative movement of the spiral elements used to create the fluid pockets and may be considered as illustrating the end view of a compressor wherein the end plates are removed and only the spiral elements are shown. Spiral elements 1 and 2 are angularly and radially offset and interfit with one another. As shown in FIG. 1a, orbiting spiral element 1 and fixed spiral element 2 form four line contacts as shown at points A-D. Fluid pockets 3a and 3b are partially defined by the walls of spiral elements 1 and 2 at line contacts C-D and A-B, as graphically illustrated by the dotted regions. Fluid pockets 3a and 3b are further defined by the end plates from which sprial elements 1 and 2 extend. When orbiting spiral element 1 is moved in relation to fixed spiral element 2 so that center O' of orbiting sprial element 1 revolves around center O of fixed spiral element 2, with a radius of O--O', fluid pockets 3a and 3b shift angularly toward the center of the interfitted spiral elements. As a result, the volume of each fluid pocket is gradually reduced as shown in FIGS. 1b-1d. Therefore, the fluid in each pocket is compressed.
Fluid pockets 3a and 3b become merged at center portion 4 to form a single pocket while passing from the stage illustrated in FIG. 1d to that illustrated in FIG. 1a. The volume of the single pocket is reduced by further revolution of orbiting spiral element 1. During the course of revolution, outer spaces which open during the state shown in FIG. 1b progressively change as shown in FIGS. 1c, 1d and 1a to form new sealed-off pockets in which additional fluid is newly enclosed and compressed. Accordingly, if circular end plates are disposed and sealed to the axial facing ends of spiral elements 1 and 2, respectively, and if one of the end plates is provided with a discharge port at center portion 4, fluid may be taken into the fluid pockets at the radial outer portion and can be discharged from center portion 4 after compression.
As mentioned above, fluid is compressed in scroll type compressors by changing the volume of the fluid pockets by orbital motion of one of the scroll elements. Fluid pockets are formed by line contacts between the spiral curved surfaces of the spiral elements and axial contacts between the end surface of the circular end plates and the axial end surface of the spiral elements. In accordance with the orbital motion of the orbiting elements, these line contacts shift along the spiral curved surface of the spiral elements to compress the fluid.
The scroll type fluid displacement apparatus is suitable for use as a refrigerant compressor. When used as a compressor, it is desirable that the scroll have sufficient mechanical strength to compress fluids under high pressure. In scrolls known in the prior art, the end plate and associated spiral element are integrally formed. However, the base or end portion of the spiral element, i.e., the portion of the element which joins the end plate, particularly the inner end portion or edge, is disposed in an area of high pressure, temperature and stress which forms at the center of the interfitting elements. Accordingly, the strength and rigidity of the inner end portion of the element is substantially reduced over time due to fatigue and deterioration. As a result, the spiral element is susceptible to developing small cracks in this area. These small cracks soon develop into larger cracks which can lead to the spiral element separating from the end plate and attendant destruction of the scroll.
The scroll type compressor is particularly suitable for use in automobile air conditioners where compact size is desirable. However, if the heights of the spiral elements are increased as a means to enlarge the displacement volume of the compressor without expanding its overall diameter, the stresses developed inside the scroll are increased. Accordingly, the above-described deterioration of the inner portion of each element is hastened.